When a transmitter and receiver are physically located near one another, the receiver needs to be isolated from the transmitter, particularly when the transmitter and receiver operate simultaneously. Isolation is needed so that the transmitter's transmitted signal does not interfere with the signal the receiver is trying to receive. The isolation may be aided to a great degree by configuring the transmitter and receiver to transmit and receive carrier signals respectively oscillating in non-overlapping frequency bands. But the use of separate transmit and receive bands seldom provides sufficient isolation when the transmitter and receiver share an antenna or use antennas located near one another. When the transmitter and receiver are near one another, the transmit signal is at such a vastly greater amplitude than the receive signal that the transmit signal can overwhelm the receiver.
The transmit signal can overwhelm the receiver in a variety of different ways. First, the transmit signal includes energy in the transmit band. If this transmit-band energy is not vastly attenuated from the receive signal without significantly attenuating the receive-band energy, then the transmit-band energy can exceed the power limits of the input circuits in the receiver. When the power limits are exceeded, regardless of the frequency, the input circuits in the receiver cannot successfully process the receive signal.
In addition, when narrowband interfering signals are near the receive band, any nonlinear processing in the receiver's input circuits, such as amplifiers and/or mixers, causes some degree of cross modulation, which produces intermodulation products in the receive band. When the transmit band energy and/or the nearby narrowband interfering signal energy are relatively high at the front end of the receiver, such receive band intermodulation products can be considerably greater than the energy in the receive signal itself.
Another way that the transmit signal may overwhelm the receiver results because the transmit signal itself often includes a small amount of energy in the receive band. This receive-band energy portion of the transmit signal often results from intermodulation due to nonlinear processing in a high-power amplifier (HPA) at the output section of the transmitter and may also result from linear amplification of out-of-band thermal noise at the HPA input. Desirably, any energy outside the transmit band, including energy falling in the receive band, is held to as low a level as possible. But inevitably, some small residual portion of receive-band energy is nevertheless present in a transmit signal. And, since the transmit signal is at such a vastly greater amplitude than the receive signal, this small residual portion of receive-band energy in the transmit signal might nevertheless exhibit a sufficient amplitude to interfere with the receive signal. Accordingly, adequate isolation has often suggested that the receive-band energy be attenuated from the transmit signal without significantly attenuating the transmit-band energy and that transmit-band energy be attenuated from the receive signal without significantly attenuating the receive-band energy.
It has been a common practice in frequency division duplex (FDD) communication systems to use a duplexer to provide the desired isolation between a transmitter and receiver. In an FDD system, transmission and reception takes place simultaneously, but in different, predetermined frequency bands. A duplexer essentially includes a “transmit” isolation filter for the transmit signal, where the transmit filter is configured to pass transmit-band energy but to attenuate receive-band energy. A duplexer also includes a “receive” isolation filter for the receive signal, where the receive filter is configured to pass receive-band energy but to attenuate transmit-band energy. A duplexer provides an added benefit of allowing the transmitter and receiver to share a common antenna. In particular, the output of the transmit filter and the input of the receive filter share a common port of the duplexer, and this common port couples to a shared antenna.
In order to be useful, the isolation filters, whether or not included in a duplexer, should exhibit low insertion loss. In other words, the transmit filter should minimally attenuate the transmit-band energy it passes, and the receive filter should minimally attenuate the receive-band energy it passes. All other design parameters remaining equal, increased insertion loss directly causes a reduced link margin, leading to a reduced radio range, reduced data communication rates, increased error rates, and/or the like.
Isolation filters should also exhibit flat responses throughout their passbands. In other words, the insertion loss should be constant over the entire passband of the isolation filter, whether for the transmit filter or the receive filter. Any rippling or other inconstancy in this response produces distortion, which again leads to reduced radio range, reduced data communication rates, increased error rates, and/or the like.
Furthermore, isolation filters should provide a narrow transition band. A narrower transition band for the transmit filter causes the transmit filter to more greatly attenuate receive-band energy without further attenuating transmit-band energy. Likewise, a narrower transition band for the receive filter causes the receive filter to more greatly attenuate transmit-band energy without further attenuating receive-band energy. An inadequately narrow transition band has conventionally lead to inadequate isolation and interference with the receive signal.
Unfortunately, improvements in one of these three design criteria (i.e., insertion loss, flat response, and narrow transition band) are usually achieved at the expense of at least one of the other two. Thus, a good duplexer having truly desirable design characteristics is difficult to obtain.
Furthermore, the problems of obtaining a good duplexer are exacerbated as transmitter power increases. As power increases, the ratio of the power of the transmit signal to the receive signal increases, making adequate isolation more difficult to achieve. And, while printed and photolithographic devices, such as surface acoustic wave (SAW) devices and film bulk acoustic resonator (FBAR) devices, may provide good low cost, low power duplexers, such devices are not currently available for transceiver applications with transmit power greater than several watts.
For higher power applications, such as cellular base stations, which may transmit at up to several hundred watts of power, conventional duplexers that adequately balance insertion loss, flat response, and narrow transition band design criteria tend to be complex metallic structures that are complicated to manufacture, and often require individual manual tuning. As a consequence, such duplexers tend to be one of the more expensive components of a transceiver.
In a multichannel time division duplex (TDD) communication system, transmission and reception do not occur simultaneously for any single channel. But multiple TDD channels may be separated only slightly in frequency from one another. Thus, at any instant, one or more channels may be transmitting while an adjacent channel is attempting to receive. And, both transmission and reception occur in every channel. Since every channel supports both transmission and reception, no isolation filtering like that used in a conventional duplexer can be effective to isolate a transmission signal from a reception signal. Accordingly, multichannel TDD communication systems suffer the same isolation problems that FDD communication systems suffer, but they begin to suffer these problems at lower transmission power levels. And, at higher power levels multichannel TDD communication systems have been deemed impractical due to such problems.
Accordingly, a need exists for a transceiver design that uses techniques other than relying on isolation filters or other than relying solely on isolation filters to isolate a transmitter and a receiver. Such a transceiver design may operate in conjunction with isolation filters, but the isolation filters may then be less complicated, smaller, and less expensive than conventional higher power isolation filters and/or duplexers. Alternatively, such a transceiver design may be used in a communication system with smaller transition bands between the transmit and receive bands. Or, such a transceiver design may be used without isolation filters altogether so as to isolate transmitter from receiver in a multichannel TDD communication system.
Prior art transceivers have attempted to implement the cancellation of a transmit signal from a receive signal, but such attempts have been met with limited success. Some such transceivers utilize transmit signals extracted upstream of a duplexer for use as a reference signal with which cancellation is performed. But the version of the transmit signal that interferes with the receive signal is influenced by duplexer distortions. Since such duplexer distortions are not accounted for in the reference signal, the resulting cancellation is less effective than it might be. To be effective, at least a portion of such cancellation should occur at RF frequencies. Unfortunately, prior art transceivers have attempted to control the processing of reference signals used in cancellation exclusively using analog components. But analog components introduce a wide variety of offsets, distortions, and inaccuracies that may be omitted in digital processing. As a result, reference signals are processed inaccurately, limited amounts of cancellation are achieved, and the character of cancellation is unsuitable for wideband transmit and/or receive signals.